Process for rejuvenating a diffusion aluminide coating

Abstract

A process of rejuvenating a diffusion aluminide coating on a component. The rejuvenation process involves treating the coating with an aqueous solution of nitric acid and phosphoric acid until at least part of the additive layer of the coating has been removed, but the diffusion zone underlying the additive layer remains. The exposed surface of the component is then re-aluminized to deposit additional aluminum to build up the additive layer to a desired thickness. While potentially useful for a variety of situations, the process is particularly applicable to a diffusion aluminide coating that has been deposited on a component to have an excessively thick additive layer, and prior to the component being returned to service.

1. A process for rejuvenating a platinum-containing diffusion aluminide coating on a component following deposition of the diffusion aluminide coating and before placing the component in service at an elevated temperature, the diffusion aluminide coating comprising an additive layer on a surface of the component and a diffusion zone below the additive layer and in a surface region of the component, the process comprising the steps of:
 treating the diffusion aluminide coating to an aqueous solution consisting essentially of nitric acid and phosphoric acid at a temperature of about 70° C. to about 80° C. until at least part of the additive layer has been removed but the diffusion zone remains, thereby establishing a treated surface of the diffusion aluminide coating;  
 depositing a platinum layer on the treated surface;  
 heat treating the component to diffuse the platinum layer into the treated surface; and then  
 aluminizing the treated surface of the component.  
 
     
     
       2. A process according to  claim 1 , wherein the aqueous solution consists of HNO 3 , H 3 PO 4 , and water. 
     
     
       3. A process according to  claim 1 , wherein the diffusion aluminide coating is treated for a duration of about 20 to about 30 minutes. 
     
     
       4. A process according to  claim 1 , wherein the aqueous solution is at a temperature of about 75° C. and the diffusion aluminide coating is treated for a duration of about 25 minutes. 
     
     
       5. A process according to  claim 1 , further comprising the steps of depositing a platinum layer on the treated surface following the treating step, and then heat treating the component to diffuse the platinum layer into the treated surface before the aluminizing step. 
     
     
       6. A process according to  claim 1 , wherein the diffusion aluminide coating is present on the component as a result of aluminizing the component after the component has been placed in service at an elevated temperature. 
     
     
       7. A process according to  claim 6 , wherein the diffusion aluminide coating is present on the component at a thickness in excess of 100 micrometers prior to the treating step. 
     
     
       8. A process according to  claim 1 , wherein the component is a gas turbine engine component, and the diffusion aluminide coating is present on the component as a result of aluminizing the component after the component was installed on a gas turbine engine, the gas turbine engine was operated, and the component was removed from the gas turbine engine. 
     
     
       9. A process according to  claim 8 , wherein the diffusion aluminide coating is present on the component at a thickness in excess of 100 micrometers after the aluminizing step and prior to the treating step. 
     
     
       10. A process according to  claim 1 , wherein the treating step removes substantially all of the additive layer and does not damage the surface region of the component. 
     
     
       11. A process for rejuvenating a platinum-containing diffusion aluminide coating on a gas turbine engine component formed of a nickel or cobalt-base superalloy, the diffusion aluminide coating comprising an additive layer on a surface of the component and a diffusion zone below the additive layer and in a surface region of the component, the process comprising the steps of:
 removing the component from a gas turbine engine after the gas turbine engine was operated;  
 aluminizing the component to form a rejuvenated diffusion aluminide coating having an additive layer with a thickness in excess of 100 micrometers;  
 before placing the component in service on a gas turbine engine, treating the diffusion aluminide coating to an aqueous solution consisting of about 50 volume percent nitric acid and about 50 volume percent phosphoric acid at a temperature of about 70° C. to about 80° C. for a duration of about 20 to about 30 minutes to remove at least part of the additive layer but the diffusion zone remains, thereby establishing a treated surface of the diffusion aluminide coating on the component;  
 depositing a platinum layer on the treated surface;  
 heat treating the component to diffuse the platinum layer into the treated surface; and then  
 aluminizing the treated surface of the component to form a second rejuvenated diffusion aluminide coating having an additive layer with a thickness of not greater than 100 micrometers.  
 
     
     
       12. A process according to  claim 11 , wherein the aqueous solution consists of HNO 3 , H 3 PO 4 , and water. 
     
     
       13. A process according to  claim 11 , wherein the aqueous solution is at a temperature of about 75° C. and the diffusion aluminide coating is treated for a duration of about 25 minutes. 
     
     
       14. A process according to  claim 11 , further comprising the steps of depositing a platinum layer on the treated surface following the treating step, and then heat treating the component to diffuse the platinum layer into the treated surface before the aluminizing step. 
     
     
       15. A process according to  claim 11 , wherein the treating step removes substantially all of the additive layer and does not damage the surface region of the component. 
     
     
       16. A process according to  claim 11 , wherein the step of aluminizing the treated surface is a vapor phase aluminiding process.